#vba #v5 #3dx
When files are moved or downloaded, ensuring their integrity is crucial. One popular method for verifying that a file has not been altered is through MD5 hashing.
What is MD5 Hashing?
MD5 (Message Digest Algorithm 5) is a cryptographic hash function that produces a 128-bit (16-byte) hash value, typically represented as a 32-character hexadecimal number. It takes an input (such as a file) and generates a unique hash. Even a tiny change in the file will result in a completely different hash.
Why is MD5 Important?
MD5 is widely used to verify file integrity. When you download a file, the provider often supplies an MD5 hash. After downloading, you can compute the hash of your copy and compare it to the original. If the hashes match, the file is intact; if not, it may be corrupted or tampered with.
How to Use MD5 for File Integrity:
1. Before Transfer: The sender computes the MD5 hash of the file and shares it.
2. After Transfer: The receiver computes the MD5 hash of the received file.
3. Comparison: If both hashes are identical, the file is unchanged.
How to Use MD5 for File Integrity
You can verify file integrity using MD5 in two common ways: with PowerShell or with VBA code.
Using PowerShell
PowerShell provides a simple command to compute the MD5 hash of a file:
Get-FileHash -Algorithm MD5 "C:\path\to\file"
This command outputs the MD5 hash, which you can compare with the expected value.
Using VBA Code
If you prefer to automate the process within Microsoft Office or other VBA-enabled environments, you can use a VBA module to compute the MD5 hash. For example, you might have a module like ComputeMD5.bas with a function to calculate the hash of a file:
On Error Resume Next
Dim Hash As String
Hash = ComputeMd5("C:\path\to\file")
If Err.Number <> 0 Then
MsgBox "Error: " & Err.Description
Else
MsgBox "MD5: " & Hash
End If
This approach is useful for integrating file integrity checks into your own automation workflows.
Limitations
While MD5 is fast and easy to use, it is not collision-resistant. For highly sensitive applications, stronger algorithms like SHA-256 are recommended. However, for basic file integrity checks, MD5 remains a practical choice.
Conclusion
MD5 hashing is a simple yet effective way to ensure files remain unchanged during transfers. By comparing hash values before and after moving or downloading files, users can quickly verify file integrity and prevent issues caused by corruption or tampering.
You can also check external files (settings, XML, TXT, etc.) of your application for unwanted modifications before using the app. This check will not prevent users from modifying the files, but if they do so, you will be able to log the issue and inform them to use an unmodified file.😉
ComputeMd5.bas
The original code has been extracted from this source and then refactored to make it more module-friendly and easier to understand.
' Checks if the specified file exists.
Private Function FileExists(FilePath As String) As Boolean
On Error GoTo ErrorHandler
FileExists = (Len(Dir(FilePath)) <> 0)
Exit Function
ErrorHandler:
FileExists = False
End Function
' Allocates a buffer for reading file data, rounding up to the nearest KB if needed.
Private Function AllocateBuffer(HFile As Integer, BlockSize As Long) As Byte()
Const KB As Long = 1024
' Adjust block size to next multiple of 1024 if file is smaller than requested block size
If LOF(HFile) < BlockSize Then
BlockSize = ((LOF(HFile) + KB - 1) \ KB) * KB
End If
Dim Buffer() As Byte
ReDim Buffer(0 To BlockSize - 1)
AllocateBuffer = Buffer
End Function
' Computes the MD5 hash of a file stream using block-wise reading.
Private Function ComputeFileHash(HFile As Integer, BlockSize As Long) As Byte()
Dim Md5Provider As Object
Set Md5Provider = CreateObject("System.Security.Cryptography.MD5CryptoServiceProvider")
Dim FileLen As Long
FileLen = LOF(HFile)
Dim NumBlocks As Long
NumBlocks = FileLen \ BlockSize
Dim RemainderSize As Long
RemainderSize = FileLen Mod BlockSize
Dim Buffer() As Byte
ReDim Buffer(0 To BlockSize - 1)
Dim i As Long
' Read and process each block
For i = 1 To NumBlocks
Get HFile, , Buffer
Md5Provider.TransformBlock Buffer, 0, BlockSize, Buffer, 0
Next i
' Process the final block
If RemainderSize > 0 Then
Get HFile, , Buffer
Md5Provider.TransformFinalBlock Buffer, 0, RemainderSize
Else
Md5Provider.TransformFinalBlock Buffer, 0, 0
End If
ComputeFileHash = Md5Provider.Hash
Md5Provider.Clear
Set Md5Provider = Nothing
End Function
' Converts a byte array to a hexadecimal string.
Private Function BytesToHex(Buffer() As Byte) As String
' Converts a byte array to a hexadecimal string
Dim HexStr As String: HexStr = ""
Dim i As Long
For i = LBound(Buffer) To UBound(Buffer)
HexStr = HexStr & Right("0" & Hex(Buffer(i)), 2)
Next i
BytesToHex = HexStr
End Function
' Main function to compute the MD5 hash of a file and return it as a hex string.
Public Function ComputeMd5(FilePath As String) As String
Const DEFAULT_BLOCK_SIZE As Long = 2 ^ 16
If Not FileExists(FilePath) Then
Err.Raise 53, , "File not found." & vbCr & FilePath
End If
Dim HFile As Integer
HFile = VBA.FreeFile
On Error GoTo ErrorHandler
Open FilePath For Binary Access Read As HFile
Dim HashBuffer() As Byte
HashBuffer = ComputeFileHash(HFile, DEFAULT_BLOCK_SIZE)
ComputeMd5 = BytesToHex(HashBuffer)
If HFile <> 0 Then Close HFile
Exit Function
ErrorHandler:
On Error Resume Next
If HFile <> 0 Then Close HFile
Err.Raise 5, , "File could not be processed." & vbCr & FilePath
End Function